WASHINGTON (CNN) -- Sen. Edward Kennedy launched a blistering election-year attack on the Bush administration's candor and honesty Monday, saying President Bush has created "the largest credibility gap since Richard Nixon."

The Massachusetts Democrat said that Iraq was never a threat to the United States and that Bush took the country to war under false pretenses, giving al Qaeda two years to regroup and plant terrorist cells throughout the world.

"Iraq is George Bush's Vietnam," Kennedy said at the Brookings Institution, a Washington think tank.

Responding to the criticism, Bush campaign spokesman Terry Holt called the veteran lawmaker the "lead political hatchet man" for Sen. John Kerry's campaign, adding that if it had been up to Kennedy, "Saddam Hussein would not be in prison but would still be in power."

A Bush campaign official called the comparison between Bush and Nixon "bizarre" and said it would only resonate with the "extreme left."

Kennedy's comments also drew a strong rebuke from Senate Majority Whip Mitch McConnell, R-Kentucky, who took to the Senate floor to denounce them as "vicious" and "outrageous."

Kennedy also said that "truth is the first casualty of policy" in the Bush administration.

"This is the pattern and the record of the Bush administration [on] Iraq, jobs, Medicare, schools, issue after issue -- mislead, deceive, make up the needed facts, smear the character of any critics," he said. "Again and again, we see this cynical, despicable strategy playing out."

Kennedy also said recent statements by former White House counterterrorism adviser Richard Clarke showed the administration's "inattention to the grave and gathering threat of terrorism before 9/11 and the president's preoccupation with Iraq."

"This misguided war in Iraq has distracted us from the real war we must win and made that war harder to win, because even as we combat terror, it has left America more and more isolated in the world," Kennedy said.

McConnell said Kennedy's remarks would be seen in Baghdad, "where those who are fighting Americans on the street can view them."

"The only one responsible for the attack on America is al Qaeda," McConnell said. "We need to focus on rooting out global terrorism by fighting the terrorists, not each other."

The Bush administration has said more than 70 percent of al Qaeda's leadership has been killed or captured and that the terrorist network is on the run.

Deception on Medicare cited

In his assault on the Bush administration's credibility, Kennedy also said the White House deliberately misled Congress about the true cost of the Medicare prescription drug bill.

"On Jan. 29, seven weeks after the bill was signed into law, the administration finally admitted that its true estimate of the cost would be $534 billion over the next 10 years -- $134 billion higher than the cost Congress was given when it approved the proposal," he said.

Rick Foster, chief actuary for the Centers for Medicare and Medicaid Services, has said that then-agency chief Thomas Scully threatened his job if he answered questions from congressional Democrats about the cost of the bill before a series of key votes last summer. Scully, who has since left the agency, denied that he threatened to fire Foster. The Department of Health and Human Services has launched an internal investigation.

Kennedy said concealment of the figures shows the administration has "no scruples."

Kennedy also criticized Bush for underfunding education programs, including the No Child Left Behind Act, an education reform measure that the president pushed through Congress with Kennedy's help.

"We called the law the No Child Left Behind Act because we meant just what it said -- no child means no child," he said. "The tragedy is that these long overdue reforms are finally in place, but the funds are not."

Responding to the criticisms on Medicare and education, Holt said that while Kennedy worked with the Bush administration on these issues, "it's a whole new game" now that Kerry is the presumptive Democratic nominee.

"This president, after years of stonewalling from the Democrats, was able to pass and sign a plan for a prescription drug benefit," Holt said. "Kennedy was part of that as well. He was one of the lead negotiators for the Democrats. This is really a stunning about-face. He seems to be walking away from his own ideas."

Kennedy was an early supporter of the bill but voted against it in its final form. He has said the plan benefited drug companies more than patients.

The Bush campaign official questioned Kerry's use of Kennedy as his "principal spokesperson."

"Most people have made up their minds long ago that Ted Kennedy is way out of the mainstream of national politics," the official said. "Kerry has set him up, and he is almost playing the role of a running mate."